A 79-year-old man was admitted because of consciousness disturbance on August 9, 2002. He had been diagnosed as having chronic myeloid leukemia in 1999, and since then, he had continued to take hydroxyurea (1500 mg/day) orally. On admission, his serum sodium concentration was as low as 119 mEq/L, while urinary sodium excretion was high. Based on the blood picture and lack of hepatosplenomegaly, we considered that the leukemia was still in the chronic phase. Because of normal blood level of the antidiuretic hormone (ADH) concentration and sufficient urine volume, the syndrome of inappropriate ADH secretion (SIADH) was unlikely, and sodium-losing nephropathy was suspected. After discontinuation of hydroxyurea, the urinary sodium excretion decreased and the patient's consciousness became clear concomitantly with improvement in the serum Na level. This patient appears to be the first case of hyponatremia caused by hydroxyurea.